1. Field of the Invention
The present invention relates to a power supply branching control apparatus and method for supplying power to electric loads in which, for a plurality of electric loads powered through a power supply switching device and a reverse connection protection device, a plurality of the reverse connection protection devices can be connected to and branched from the power supply switching device that is shared by the reverse connection protection devices.
2. Description of the Related Art
For example, for in-vehicle electric loads, a plurality of power supply switching devices are placed between a DC power supply that is an in-vehicle battery and numerous in-vehicle electric loads, one of the power supply switching devices selectively supplying power to one or more of the in-vehicle electric loads. For example, according to FIG. 6 in Patent Literature 1, “In-vehicle power supply distribution apparatus”, a field-effect transistor 44A that is an intelligent power switch is used as a power supply switching device in place of an output contact of a magnetic relay. On the other hand, a technique is widely used in which a reverse connection protection diode is connected in series to an electric load powered by a DC power supply so that load current will not flow when the power supply is connected in wrong polarity. A field-effect transistor as a reverse connection protection device is widely used in order to reduce voltage drop and temperature increase caused by a reverse connection protection device in normal operation. Note that the field-effect transistor mentioned herein includes a P-channel type and an N-channel type, in any case of which, as far as the power supply is connected in correct polarity, the field-effect transistor is caused to close so as to be conductive in the same direction as the conduction direction of an internal parasitic diode.
For example, according to FIG. 4 in Patent Literature 2, “Power supply reverse connection protection circuit”, in an ECU 45 powered by a battery 3, an N-channel type FET 21 is provided on a power supply wire 15 connecting a power supply terminal 5 connected to the positive terminal of the battery 3 and a control circuit 13 to be powered, with the anode of a parasitic diode D1 of the FET 21 on the power supply terminal 5 side, and furthermore, an N-channel type FET 22 is provided on the downstream side of the FET 21, with the cathode of a parasitic diode D2 of the FET 22 on the FET 21 side. Then, when an ignition key switch 9 is turned on with the battery 3 correctly connected, charge pump circuits 43, 47 powered from the drain side of the FET 21 turns on the FETs 21, 22, causing the control circuit 13 to be powered by the battery 3. On the other hand, when the battery 3 is connected in reverse polarity, the FETs 21, 22 are turned off and reverse current is interrupted by the parasitic diode D1.
When powered by the battery 3 after the FET 22 is turned on, the control circuit 13 starts operation by outputting a drive signal Sd so that the FETs 21, 22 will be kept on even when the ignition key switch 9 is turned off. Also, although not shown, the control circuit 13 monitors the voltage of a signal input terminal 11 in order to detect the on/off state of the ignition key switch 9. Then the control circuit 13 detects that the ignition key switch 9 is turned off, based on the voltage of the signal input terminal 11, and then, when a preprocessing for operation stop, such as data migration, ends and a condition for allowing operation stop is satisfied, stops outputting the drive signal Sd.    [Patent Literature 1] JP-A-10-126963 (FIG. 1, Abstract, FIG. 6, [0025])    [Patent Literature 2] JP-A-2007-082374 (FIG. 4, Abstract, FIG. 1, [0056], [0057])
According to Patent Literature 1, a field-effect transistor that is an intelligent power switch is used as a power supply switching device. Accordingly, when overcurrent flows in the power supply switching device due to abnormality of an electric load or load wiring, the power supply switching device can be opened for protection. However, when an in-vehicle battery is connected in wrong power-supply polarity, a problem is caused in which short circuit current flows in a parasitic diode generated in the field-effect transistor, thereby damaging the power supply switching device or a related device connected thereto by overheat. On the other hand, according to Patent Literature 2, a reverse connection protection device and a power supply switching device are used as one composite switching device, resolving the problem of connection in wrong power-supply polarity. However, a problem is caused in which providing one composite switching device to every one of a plurality of electric loads may be uneconomical.